Commercial aircraft currently carry fresh water for the flushing of vacuum operated toilets in these aircraft. Moreover, the lavatory basins also use freshwater which, by its use in the handwash basin becomes so called “gray water”. Thus, in the passenger lavatories substantial quantities or volumes of gray water are produced, which conventionally are not used for any further purpose. The inventors have discovered that following a simple treatment or processing of this gray water it would be quite suitable for use as toilet flushing water.